The present invention relates to compositions for producing foundry molds, and more specifically to foundry mold compositions incorporating a modifier for improved foundry molds.
In the casting of molten metals and alloys for various foundry applications, it is customary to employ foundry molds having a configuration conforming to the shape of the desired casting. Foundry molds made of sand may be used to form the outside of castings or may be cores, positioned inside the mold to shape the inside of the casting. Foundry molds may be constructed from compositions that include foundry sand as the major component in combination with a mineral clay and water. Supplemental additives may include ground bituminous coal, lignite, leonardite, pregelatinized starches, cellulose and other conventional additives may also be present in minor amounts. The foundry mold composition may be produced by introducing foundry sand, water, any supplemental additions, and the mineral clay into a mixing apparatus, such as a muller. The mixing of these constituents may be performed to an extent that the particles of the foundry sand are coated by the mineral clay component. The supplemental additives and the mineral clay may be added as a single mixture to the foundry sand and water in a mixing apparatus so that the sand particles may be coated with the mineral clay. Thereafter, the composition from the muller may be introduced to a flask or confining structure incorporating a pattern configured to correspond to the desired configuration of the metal or alloy casting. The composition may be consolidated within the pattern to obtain the required integrity, and then the pattern may be removed to render the foundry mold ready for use in producing a metal or alloy casting.
A significant property or characteristic of foundry mold compositions may be “durability” or the resistance of the mold and specifically the mineral clay component to thermal degradation at the elevated temperatures encountered during conventional hot metal and alloy casting. Specifically, it may be advantageous for the mold composition to be resistant to thermal degradation at temperatures ranging from 400° F. to 1200° F. Mineral clays may burn-out or lose their desired properties progressively as temperatures increase within this range. Sodium bentonite may exhibit greater durability compared to calcium bentonite in mold compositions. A second significant property may be the “dry strength” or the energy required to remove the solidified casting from the mold. In conventional foundry mold compositions containing blends of sodium bentonite and calcium bentonite, the dry strength property of the mold may be enhanced by increased amounts of calcium bentonite, which serves to reduce dry strength and facilitates easier removal of the casting from the sand mold. The calcium bentonite, however, may result in degradation of the durability of the mold. A third significant property may be “moldability” or the measurement of apparent cohesion between sand grains of the mold composition. Mold compositions deficient in this property may stick in hoppers and transfer equipment, which may be detrimental to the entire casting process. Mold compositions containing calcium bentonite as all or part of the mineral clay content may exhibit improved moldability, particularly when the water content of the mold composition increases. A fourth significant property may be “hot strength” or the ability of the mold composition to maintain its integrity at the mold composition/molten metal interface during and following pouring of the metal, usually at temperatures of 1500° F. and above. All of the additional, desired foundry mold properties may be attributed to equally by sodium bentonite or calcium bentonite or enhanced by sodium bentonite in the mold composition.
The additional properties include “green strength” or the strength or integrity of the mold prior to pouring of the molten metal into the mold. “Wet tensile strength” is the resistance of the mold to degradation due to transient shocks or jolts. “Hot deformation” is the ability of the mold to maintain dimensional stability during hot metal casting so that required dimensional tolerances are achieved with respect to the solidified casting. “Permeability” is defined as that property of a sand mold which allows gas to pass through it: The venting qualities of molds and cores depend upon this property. Permeability is influenced by the size, shape and distributing of the grains of the sand, the type and quantity of bonding material, the density to which the sand is rammed and the moisture content. “Friability” is a measure of the abrasion resistance of a sand mold. A friable sand is a sand that is not able to withstand the erosive flow of the molten metal. It will lose sand grains to the moving stream, and will be subject to producing erosion and inclusion defects. Generally, friability is inversely related to compactability; the lower the compactability, the higher the friability.